Broad, fan-shaped lamellipodia extend out from the A-549 lung carcinoma epithelial cells and are sporadically retracted in a flowing motion. Cytoplasmic streaming occurs each time one of the surface extensions is formed or drawn back. The cytoplasm in the central region of the cell is called plasmasol and is much more fluid than the cytoplasm (plasmagel) at the periphery of a lamellipodium. As a lamellipodium grows, plasmasol flows into the surface structure and assumes a more gel-like consistency around the margin of the surface extension. When a lamellipodium is retracted, the plasmagel liquefies and streams back into the center of the cell. The physical transformations of cytoplasm are the manifestation of characteristic modifications of cross-linking actin filaments present in the cell.

The plasmagel of a cell is transparent and free from organelles, as can be observed during the high speed playback of this time-lapse sequence. Cell nuclei remain relatively centralized and the numerous mitochondria present are concentrated around the much larger organelle. In the A-549 line and other epithelial cells, keratin filaments play an important role in maintaining the proper positioning of nuclei and are also crucial in enabling animal cells to withstand mechanical stress. Keratin filaments becomes particularly important when the cells, which are naturally prone to the formation of layers, join together to produce epithelial sheets. In such instances, the filaments are frequently found anchored to specialized cell junctions called desmosomes.

The tendency of epithelial cells to form connections with other epithelial cells is pronounced in the featured video. Less than half of the field of view is filled with cells as the sequence initiates, but by the end an almost contiguous monolayer has formed. The cell-cell linkages between epithelial cells, however, do not necessarily prevent them from sometimes shifting positions with neighboring cells. In fact, migrations of complete sheets of epithelial cells can be generated via the active repositioning of the cells it contains.